[CylobBot] Modular analogue synthesis as an extension of a digital system, part 3

**Joyrex** · May 28, 2012

Sending control voltages from a soundcardThere is nothing complicated about this method of control – audio signals consist of the kind of electrical modulation which is also harnessed by modular synths. Sending continuous values instead of a typical rapidly changing audio waveform can perform the role of controlling input parameters.Unfortunately, most soundcards have in place high pass filters which remove very low frequencies, so continuous DC signals aren’t stable, and any attempt to use them results in the signal dropping back to nothing. Some soundcards don’t have this filtering though, and there is one made especially for the purpose of controlling modular synths: Expert Sleepers’ ES3. It takes an optical ADAT input, gives 8 audio outs, and comes in a eurorack format ready for screwing into a rack with the modules themselves.Here is a test SuperCollider patch along with the resulting audio. The patch is outputting 40 discrete values from 0.0 to 1.0, and this output is going into the FM input of a TipTop Audio Z2040 Low Pass Filter with its resonance turned up to self-oscillation. The base frequency of the filter and the amount of modulation can both be adjusted on the filter module. It can be heard that audio values from 0.0 to 1.0 are enough to cover the full range of frequencies that would be generally required.

Demo: pitch steps

{var noOfSteps = 40, dseq, trig, demand;dseq = Dseq(Array.interpolation(noOfSteps, 0.0, 1.0), inf);trig = Impulse.ar(4);demand = Demand.ar(trig, 0, dseq);Out.ar(10, demand);}.play

A patch like this could form the basis of a calibration routine which would analyse the pitch of audio resulting from certain control values, and create a map from pitch to control voltage.EnvelopesSo now the principle has been established, thoughts may naturally turn to a more interesting use of control signals. My first ambition for this project was to create analogue drum sounds. Having used a friend’s Octave Kitten synth, I fell in love with its filter: it sounds rich, creamy and powerful, capable of making both delicious melodic synth sounds, and put into self oscillation, thundering and punchy percussion. I read that the TipTop Z2040′s filter chip is from the same family, and although it sounds much better engineered than the Kitten, the resemblance is noticable and it has many of the same sound qualities.The basis of my drum sound patch is to simply play two CV control envelopes together in order to change the frequency of a resonating filter. The output from the filter is then passed through a SC audio envelope. Both the control envelopes consist of simple attack-release shapes. The first is intended for the overall shape (i.e. the oomph of a 909 kick) and the second for very tight clicks (such as the high frequency initial hit sound of the 909 kick.) Here is the sound of making such a kick drum: